1. Field of the Invention
The present invention relates to an ozone generating apparatus.
2. Description of the Prior Art
It is known that ozone can be generated by discharge and can be used in various fields as an oxidizing agent or as a bactericide. Recently, applications of ozone have increased, especially in the field of elimination of pollution, ozone being used for treatment of sewage, industrial drainage and nitrogen oxides NO.sub.x in effluent gas.
FIG. 1( a) is a schematic view of a conventional ozone generating apparatus where a glass discharge tube 13 is disposed in the center of a metallic cylinder 12 and a metallic electrode 14 is adhered or vapor metallized on an inner surface of the glass discharge tube 13 and is connected to a high voltage terminal 15.
A power source 1 is connected to the high voltage terminal 15 and the metallic cylinder 12 on its outer surface so as to apply a sinusoidal waveform voltage having a commercial or high frequency. Oxygen in the air fed from one end of the metallic cylinder 12 is converted to ozone by the discharge in the gap between the metallic cylinder 12 and the glass discharge tube 13.
FIG. 2 is a circuit diagram of the power source system of the conventional ozone generating apparatus.
FIGS. 3( a), (b) are schematic operating waveforms of power voltage V and current i of the ozone generator.
In FIG. 2, the reference 1 designates the power source for driving the ozone generator (such as a power source having commercial frequency); 2 designates a boosting transformer; 3 designates an ozone generator and 4 designates a reactor for power-factor improvement.
When the power voltage is the sinusoidal waveform voltage V.sub.(t) of FIG. 3(a), the ozone generator 3 is an equivalent capacitor whereby the phase gains .pi./2 (rad) from the power voltage as shown in FIG. 3(b). Accordingly, the current waveform is significantly changed during the discharge period T.sub.E and the current i.sub.(t) as a part of the sinusoidal waveform is fed during the non-discharge period T.sub.D.
The ozone generator can be considered equivalent to the series circuit of a capacitor C.sub.g formed by the glass discharge tube 13 and a capacitor C.sub.a formed by a gap between the metallic cylinder 12 and the glass discharge tube 13 as shown in FIG. 1( b). In FIG. 1( b), when the discharge occurs in the gap, the capacitor C.sub.a is considered as forming a short-circuit and only capacitor C.sub.g remains in the equivalent circuit. This is schematically shown as turning on the switch S.
The operation of the conventional ozone generator will now be described. The ozone generator is considered as a series circuit of the capacitors C.sub.g and C.sub.a wherein C.sub.a &lt;&lt; C.sub.g, in general. As shown in FIG. 1(b), the voltage applied to the ozone generator is shown as V, the partial voltage for the capacitor C.sub.g is shown as V.sub.g and the partial voltage for the capacitor C.sub.a is shown as V.sub.a.
When the sinusoidal waveform voltage V is applied to the ozone generator, as shown in FIG. 4, the terminal voltage V.sub.a of the capacitor C.sub.a reaches the positive discharge voltage V.sub.s at the time t.sub.1 and the discharge in the gap occurs to provide O(V) of V.sub.a. Prior to the discharge, the terminal voltage V.sub.g of the capacitor C.sub.g which is formed by the glass discharge tube is not substantially changed under the relation C.sub.a &lt;&lt; C.sub.g, as shown by the dotted line.
However, when the discharge occurs as the short-circuit of C.sub.a at the time t.sub.1, all of the power voltage V is applied to C.sub.g to provide V(t.sub.1) = V.sub.g (t.sub.1) at the time t.sub.1 and V.sub.g rises to V(t.sub.1) at the time t.sub.1 as shown by the dotted line. The discharge is finished in a moment and the voltage is again applied to C.sub.a.
The change of the voltage V of the ozone generator appears substantially as the change of the terminal voltage V.sub.a of C.sub.a under the relation of C.sub.a &lt;&lt; C.sub.g and V.sub.g is not substantially changed. Accordingly, the change of V.sub.a is substantially the same as that of V and the discharge occurs at the time t.sub.2 in V.sub.a = V.sub.s. At the time t.sub.2, the voltage V.sub.g becomes V(t.sub.2) = V.sub.g (t.sub.2) whereby V.sub.g rises as shown by the dotted line. The phenomenon is repeated until the time t.sub.5.
After the time t.sub.5, V.sub.a changes substantially the same as V. However, V.sub.a .gtoreq. V.sub.s is not realized until the time the maximum value of V is reached and V.sub.a falls similar to the change of V until the time t.sub.6. During this period, V.sub.a is changed from positive through zero to negative. At the time t.sub.6, V.sub.a = -V.sub.s is equal to the negative discharge voltage. At the time t.sub.6, the discharge in the negative side occurs to give V.sub.a = 0.
At the time t.sub.5, V.sub.g becomes V.sub.g = V(t.sub.5) and then V.sub.a is kept at a substantially constant value. However, at the time t.sub.6, when the discharge occurs for C.sub.a to give V.sub.a = 0, V.sub.g suddenly falls as shown by the dotted line because V(t.sub.6) = V.sub.g. After the time t.sub.6, the same condition is repeated to give V.sub.a = -V.sub.s at the times t.sub.7, t.sub.8, t.sub.9 and t.sub.10 and the discharges for C.sub.a occur at these times to change V.sub.g as shown by the dotted line.
Accordingly, when the ozone generator is used by applying a sinusoidal waveform voltage, the following conditions are realized.
1. In one cycle period of the voltage T.sub.o, the discharge phenomenon maintaining period is 2T.sub.E from t.sub.1 to t.sub.5 and from t.sub.6 to t.sub.10 as shown in FIG. 4 and the discharge ceasing period is 2T.sub.D from t.sub.5 to t.sub.6 and from t.sub.10 to t.sub.11. Thus, the discharge phenomenon maintaining period is only about 50% of one cycle period.
2. The voltage V.sub.a is changed under substantially the same condition as that of V because of a constant of .+-. V.sub.s of the discharge voltage in the gap and the fact that C.sub.g &gt;&gt; C.sub.a. Accordingly, the discharge interval is short around the zero point of the voltage V wherein dv/dt is high and the discharge interval increases and is longest around the maximum value of the voltage V wherein dv/dt is low.
Thus, t.sub.s &lt; t.sub.l in FIG. 4.
The ozone generator operates, as stated above, in one cycle period of the voltage applied by the power source whereby ozone is generated. The ozone generating rate is substantially proportional to the power fed by the power source when the conditions of the ozone generator are constant.
The power fed from the power source to the ozone generator, that is the discharge power W caused by the discharge of the glass discharge tube, is given by the equation ##EQU1## wherein .omega.= 2.pi.f, when the average discharge current I.sub.dm is fed and the sinusoidal waveform voltage having the frequency f[Hz.] is applied from the power source.
The average discharge current I.sub.dm is given by the equation ##EQU2## where the discharge area of the ozone generator is S [cm.sup.2 ].
Accordingly, the discharge power W is given by the equation ##EQU3## wherein E.sub.b is a constant determined by a characteristic of the ozone generator 3 and C.sub.g and C.sub.a are equivalent capacitors of the ozone generator shown in FIG. 1(b).
As stated above, the discharge power of the ozone generator which is a main factor of the ozone generating rate is proportional to the frequency and voltage of the power source for driving the ozone generator. Accordingly, in the conventional ozone generating apparatus operated by a commercial frequency power source, the output voltage of the power source is changed by switching taps of a secondary side of a transformer or a voltage controlling device (not shown) for controlling the ozone generating rate.
However, the conventional ozone generating apparatus has the following disadvantages.
1. The voltage applied to the ozone generator must have a waveform which changes dependent upon the time to cause the discharge. When the voltage waveform is a sinusoidal waveform as is conventional, the discharge period is 2T.sub.E during one cycle period and the non-discharge period is 2T.sub.D as shown in FIG. 3 and FIG. 4. On the other hand, the power fed to the ozone generator is usually proportional to the maximum value of the voltage applied thereto.
2. However, when the power P.sub.o is fed during the period 2T.sub.E which is realized by subtracting 2T.sub.D from T.sub.o as shown in FIG. 3 and FIG. 4, the discharge power P.sub.o is generated during the short period whereby heat in a concentrated condition is generated during the short period. Accordingly, the yield of ozone is decreased because of the rising temperature of the molecules in the gap and the discharge tube may be damaged because of the thermal and mechanical stress of the glass discharge tube for the ozone generator. Accordingly, in order to prevent these difficulties, the rated power of the discharge tube should be decreased in the case of operation by application of a sinusoidal waveform voltage.
3. Moreover, in the case of operation by application of a sinusoidal waveform voltage, dV/dt of V(t) is changed during operation and the discharge voltage V.sub.s in the gap is constant. Accordingly, the frequency for repeating the discharge in the initial discharge period near the time t.sub.1 and t.sub.6 is high and the frequency gradually decreases. The power is concentrated near the zero point of the power voltage thereby decreasing the yield of ozone and increasing the thermal and mechanical stress for the glass discharge tube as above-mentioned.
4. The ozone generator is an equivalent capacitor load to give a low power factor. Accordingly, it is necessary to connect a reactor for power factor compensation as shown in FIG. 2 which requires a capacity for the reactive power KV.sub.A of the ozone generator.
5. In conventional ozone generating apparatus operated by a commercial frequency power source, the voltage applied to the ozone generator is changed by switching taps of the secondary side of a transformer or a voltage controlling device in order to control the ozone generating rate. Accordingly, in the case of switching the taps of the secondary side of the transformer, the ozone generating rate cannot be finely controlled. In the case of the voltage controlling device, a large size auxiliary device such as an induction voltage controlling device is required depending upon the increase of capacity of the ozone generating apparatus.
6. In the case of a commercial frequency power source, the frequency is fixed. The discharge current per unit area is not changed under the application of a constant voltage. In order to increase the discharge current per unit area, the voltage should be increased. Accordingly, a large size apparatus including suitable insulation of the transformer is required.